1. Field of the Invention
The present invention relates to a highly active supported metallocene catalyst system, and particularly to a supported catalyst for olefin polymerization and the copolymerization of ethylene-1-hexene using a silica-supported methylaluminoxane/ziroconocene catalyst.
2. Description of the Related Art
A copolymer of ethylene with an α-olefin, including 1-hexene, is called a linear low-density polyethylene (LLDPE). The incorporation of α-olefin into the polymer backbone introduces side chain branching and structural defects. LDPE has density, crystallinity, melting behavior, processing characteristics, and thermal, rheological and mechanical properties that significantly differ from those of low-density polyethylene (LDPE) and high-density polyethylene (HDPE). Consequently, LLDPE has a series of applications superior to those of LDPE and HDPE.
LLDPE joined the polyethylene family after LDPE and HDPE. It can be synthesized using the multi-site conventional Ziegler-Natty (Z-IN) catalysts, as well as the so-called single-site metallocene catalysts. Unlike Z-N catalysts, metallocene catalysts have marked structural variations, which are effected through bridge modifications and substitutions in the cyclopentadienyl ligand and its analogues. Particularly in solution polymerization, they show much higher activity than Z-N catalysts. They produce ethylene homo- and copolymers having narrow molecular weight distribution (MWD) (polydispersity index 2) and uniform copolymer composition distribution (CCD) in solution polymerization, usually at high co-catalyst to catalyst ratios. Also, metallocenes, because of their ability to undergo remarkable structural variations, can regulate co-monomer-introduced branch distribution, intra-chain microstructures, and structural/enchainment defects of ethylene α-olefin (LLDPEs) in a highly versatile fashion.
Much research continues worldwide into metallocene-catalyzed olefin polymerization to make the production of polyolefins a dynamic technology-driven industry. However, several challenges have to be overcome to develop industrial-grade supported/heterogenized metallocene catalysts. These include the following: (i) maintain the single-site characteristics of metallocenes upon heterogenization; (ii) overcome the significant drop in catalyst activity; (iii) prevent catalyst leaching (which causes severe reactor fouling, and damages polymer particle morphology); and (iv) eliminate the separate feeding of the methylaluminoxane (MAO) co-catalyst (which gels and degrades during storage, and which is very costly).
Metallocenes can be generally supported using several immobilization procedures. It turns out that silica/methylaluminoxane (MAO) co-catalyst/zirconocene, in general, offers higher catalyst activity than the remaining routes. However, MAO forms gels and degrades during co-catalyst feeding. It would be desirable to provide an improved method of producing ethylene copolymers using silica and methylaluminoxane (MAO) supported metallocene catalyst that permits active center distribution and higher copolymerization activity.
Thus, a supported catalyst for olefin polymerization solving the aforementioned problems is desired.